Traditionally, communications with ships on the high seas was conducted by morse code or by via radio communications when ships were within range of a receiving station. The start of the space age in 1957 saw the gradual introduction of communications satellites, although initially, large receiving antennas were required.

However, despite its slow pace, smaller receiving equipment was developed and by the late 1970s, it was evident that satellite communications could be used for maritime purposes. In this article, the heritage systems that opened the way for current maritime communications systems will be reviewed.

Marisat
The first maritime communications system was the Marisat system owned by Comsat General Corporation and leased to the U.S. Navy and then, on February 19, 1976, the Marisat-1 satellite was launched.

Built by Hughes, using the type HS-356 platform, the 655kg. satellite carried five transponders that operated in the C-band, 1.6/1.5GHz and 300/200MHz bands, to provide telephone, telex and data communications between naval ships and shore stations. The satellite was followed by two more satellites to complete a global system.

With the introduction of the Fltsatcom series of satellites, Marisat-1 and -3 were eventually handed over to Inmarsat, where they were repurposed for civilian communications.

Marisat-2 had a more interesting career. In 1991, the satellite was relocated to 178 degrees West where it stayed until 1996. The satellites orbit had a slight inclination of 3 degrees, which gradually increased to 13 degrees.

This made it ideal for communication services for the Amundsen-Scott South Pole Station was out of reach from geostationary satellites. The inclined orbit provided about 6 hours of communications each day. To provide a direct link to the U.S., the location of the satellite was changed from over the Indian Ocean to the Atlantic Ocean at 33.9 degrees West.

On October 29, 2008, after 32 years of service, the longest for any commercial satellite to date, it was retiredfrom active service and boosted into an orbit about 200km higher than its operational slot.

Marecs
In 1973, the European Space Agency (ESA) initiated the development of a maritime satellite system named MAROTS, based on the experimental OTS communications satellite. Following design changes, the program was later renamed Marecs and two satellites were launched into a geostationary orbit on December 20, 1981, as Marecs-1, and Marecs-B2 on November 10, 1984. The latter was a replacement for the original Marecs-B satellite, which failed to orbit on September 10, 1982.

The satellites, which had been built by British Aerospace and had a mass of 497kg., were equipped with two transponders which operated in the C- and 1.6/1.5GHz bands. They provided communication facilities between ship and shore based stations as well as between ships themselves.

The Marecs-B satellite incorporated some design improvements to make the circuitry less susceptible to electrostatic interference.

The Marecs-1 satellite was handed over to Inmarsat on May 1, 1982, after initial problems that had been experienced due to the electrostatic discharges on the satellites external surface during geomagnetic storms were overcome. In 1991, Marecs-1 was moved to 22 degrees East. Marecs-B2 was later shifted to 26 degrees West and then to 55 degrees West.

Inmarsat
In 1979, the United Nations International Maritime Organization (IMO) established the International Maritime Satellite Organization (Inmarsat), for the purpose of establishing a satellite based communications network for maritime use. It was a not-for-profit and self-funding international organization and 79 countries eventually joined.

From the start, it used the designation Inmarsat and it began full operations in 1982. Over time, the organization also began to provide services to aircraft and global portable users and, to reflect that, its name was changed into International Mobile Satellite Organization.

In 1999, the organisation was converted into a private company registered in the United Kingdom. The conversion created the commercial Inmarsat plc company as well as a regulatory body that retained the name of International Mobile Satellite Organization.

Inmarsat plc continues to provide communications services to the maritime sector but has now diversified into providing telephony and data services to users worldwide, via portable or mobile terminals which communicate to ground stations through a number of geostationary telecommunications satellites. It also provides communications services to a range of governments, aid agencies, media outlets and businesses with a need to communicate in remote regions or where there is no reliable terrestrial network.

With the transfer of the Marisat and Marecs satellites to Inmarsat, there was never an Inmarsat 1 series of satellite.

The Inmarsat 2 series of satellites had been built by British Aerospace and were based on the Eurostar 1000 platform. They had a mass of 690kg. and operated in the C-band for satellite/shore communications and the 1.6/1.5GHz band for satellite/ship transmissions. The capacity was for 150 voice channels that were serviced by four transponders.

The Inmarsat-3 series of maritime communications satellite were built by Astro Space using the AS4000 platform.The 2064kg. satellites were fitted with two transponders operating in the C-band and the L bands as well as a GPS navigational instrument.

Inmarsat currently owns three Inmarsat 4 series satellites, while three Inmarsat 5 series satellites will be launched starting in 2014.

About the authorJos Heyman is the Managing Director of Tiros Space Information, a Western Australian consultancy specializing in the dissemination of information on the scientific exploration and commercial application of space for use by educational as well as commercial organisations. An accountant by profession, Jos is the editor of the TSI News Bulletin (http://tiros.zarya.info/) and is also a regular contributor to the British Interplanetary Societys Spaceflight journal as well as to SatMagazine as a Senior Contributing Editor.